C# List of objects, how do I get the sum of a property

Question

I have a list of objects. One property of the individual object entry is amount. How do I get the sum of amount?

If my list was of type double I may be able to do something like this:

double total = myList.Sum();

However I want to something similar to this, yet this syntax is incorrect.

double total = myList.amount.Sum();

How should I go about accomplishing this? I would love to use the Sum function if possible instead of looping through and calculating the value.

Answer 1

using System.Linq;

...

double total = myList.Sum(item => item.Amount);

Answer 2

如果您需要在符合特定条件的物品上进行操作...

double total = myList.Where(item => item.Name == "Eggs").Sum(item => item.Amount);

Answer 3

另一种选择：

myPlanetsList.Select(i => i.Moons).Sum();

Answer 4

Here is example code you could run to make such test:

var f = 10000000;
var p = new int[f];

for(int i = 0; i < f; ++i)
{
    p[i] = i % 2;
}

var time = DateTime.Now;
p.Sum();
Console.WriteLine(DateTime.Now - time);

int x = 0;
time = DateTime.Now;
foreach(var item in p){
   x += item;
}
Console.WriteLine(DateTime.Now - time);

x = 0;
time = DateTime.Now;
for(int i = 0, j = f; i < j; ++i){
   x += p[i];
}
Console.WriteLine(DateTime.Now - time);

The same example for complex object is:

void Main()
{
    var f = 10000000;
    var p = new Test[f];

    for(int i = 0; i < f; ++i)
    {
        p[i] = new Test();
        p[i].Property = i % 2;
    }

    var time = DateTime.Now;
    p.Sum(k => k.Property);
    Console.WriteLine(DateTime.Now - time);

    int x = 0;
    time = DateTime.Now;
    foreach(var item in p){
        x += item.Property;
    }
    Console.WriteLine(DateTime.Now - time);

    x = 0;
    time = DateTime.Now;
    for(int i = 0, j = f; i < j; ++i){
        x += p[i].Property;
    }
    Console.WriteLine(DateTime.Now - time);
}

class Test
{
    public int Property { get; set; }
}

My results with compiler optimizations off are:

00:00:00.0570370 : Sum()
00:00:00.0250180 : Foreach()
00:00:00.0430272 : For(...)

and for second test are:

00:00:00.1450955 : Sum()
00:00:00.0650430 : Foreach()
00:00:00.0690510 : For()

it looks like LINQ is generally slower than foreach(...) but what is weird for me is that foreach(...) appears to be faster than for loop.